Patients suffering from reduced renal function or renal failure often have to undergo hemodialysis treatments. During dialysis, blood is withdrawn from the patient and is circulated through a hemodialysis machine. The machine removes toxic waste products and returns the purified blood to the patient. Typically, dialysis treatments are performed three times a week for the duration of a patient's life unless a kidney transplant procedure occurs. To successfully undergo hemodialysis treatment, blood must be circulated through the hemodialysis machine at 150 to 600 ml/minute or higher flow rate for about 3-4 hours. Blood flow from the venous system is believed to be inadequate to meet the required flow rate and repeated punctures of large arteries are not feasible. Therefore, native fistulas are often created to provide blood flow access for the hemodialysis machines.
If native fistulas are unavailable or cannot be used for hemodialysis, then vascular grafts, typically made from expanded polytetrafluoroethylene (ePTFE) tubes, are surgically placed between an artery and a vein (ePTFE AV grafts). This procedure is especially useful in patients who do not have blood vessels that will support the construction of a more traditional primary native fistula in the forearm. The ePTFE AV grafts, which are extruded, are favored over textile AV grafts, which are woven, knitted, braided or otherwise formed, for several reasons, including the unique microstructure characterized by nodes and fibrils imparted to the ePTFE grafts, which facilitates tissue ingrowth while simultaneously providing a fluid-tight conduit through which blood can flow; and the ability to provide a graft with a relatively thin wall while retaining necessary strength characteristics.
Expanded polytetrafluoroethylene AV grafts are extensively used for hemodialysis treatments as AV bridge fistulae due, at least in part, to the hemocompatibility advantage of the ePTFE material over other materials (such as polyurethane). However, one potential drawback in using ePTFE AV grafts is that they cannot be used safely to withdraw blood for hemodialysis until about 14 days post-implant. This is believed to be due to the non-elastomeric nature of ePTFE, which cannot self-seal upon puncturing. Thus, in the interim, other means of dialysis must be employed (e.g., hemodialysis catheters, etc.). After 14 days, there is typically sufficient tissue ingrowth into the ePTFE surface to act as a sealant layer, and therefore the graft can seal the puncture wound created by removal of the dialysis needle. However, such sealing requires a combination of pressure and hemostasis, which does not lend to uniformity due to the many variables present during such procedures (dialysis technician/nurse skill level, operating conditions, etc.). It is therefore preferable to have a sealing mechanism for an ePTFE vascular graft that is not dependent on hemostasis and the attendant variables associated therewith and which will seal immediately upon implantation so that additional methods of dialysis do not have to be employed.
Accordingly, various sealing techniques, such as placing a layer of elastomeric sealant on ePTFE, and composite structures have been shown or described to provide immediate self-sealing properties to an ePTFE AV graft. Examples of various types of elastomeric sealants, ePTFE grafts, self-sealing grafts, and composite grafts include those disclosed in the following U.S. patents: U.S. Pat. No. Re. 31,618, U.S. Pat. Nos. 4,604,762; 4,619,641; 4,731,073; 4,739,013; 4,743,252; 4,810,749; 4,816,339; 4,857,069; 4,955,899; 5,024,671; 5,061,276; 5,116,360; 5,133,742; 5,152,782; 5,192,310; 5,229,431; 5,354,329; 5,453,235; 5,527,353; 5,556,426; 5,607,478; 5,609,624; 5,620,763; 5,628,782; 5,641,373; 5,665,114; 5,700,287; 5,716,395; 5,716,660; 5,800,510; 5,800,512; 5,824,050; 5,840,240; 5,843,173; 5,851,229; 5,851,230; 5,866,217; 5,897,587; 5,904,967; 5,910,168; 5,931,865; 5,976,192; 6,001,125; 6,036,724; 6,039,755; U.S. Pat. Nos. 6,042,666; 6,056,970; 6,080,198; 6,099,557; 6,203,735 U.S. Pat. Nos. 6,261,257; 6,267,834; 6,287,337; 6,319,279; 6,368,347; 6,416,537; 6,428,571; 6,534,084; 6,547,820; 6,589,468; 6,627,402; 6,712,919; 6,716,239; 6,719,783; 6,790,226; 6,814,753; 6,827,737; 6,863,686; 6,926,735; 7,115,220; 7,147,617; 7,244,271; 7,452,374; 7,510,571; 7,560,006; 7,828,833; and 7,879,085, each of which is incorporated by reference as if fully set forth herein.
Before accessing an ePTFE AV graft for hemodialysis, a blood flow check through the graft is normally conducted by feeling the pulse through the graft by gently touching the surface of the skin. The ability to feel the pulse through the graft is generally defined as “palpability.” Most commercial ePTFE vascular grafts provide good palpability; however, when a layer of elastomeric sealant is placed on the surface of an ePTFE substrate, the palpability of the graft may be compromised if the layer is too thick. Another potential drawback in using ePTFE AV grafts for hemodialysis is that when implanted, there may be a tendency for the graft to form a kink at the loop site. Examples of a typical loop site is shown in FIG. 1A (forearm loop AV graft 2, from the brachial artery to the basilic vein) and 1B (thigh loop AV graft 4, from the femoral artery to the femoral vein). Kinking of the graft at the loop site may occlude blood flow, in which case immediate medical intervention would be required. Clearly, such intervention is strongly disfavored as the likelihood of adverse outcomes are increased. Unfortunately, it has been discovered that ePTFE grafts coated with elastomeric sealant or otherwise formed to address the problem of sealing can easily form kinks, presumably due to the stiffness of the graft at the loop region.
One other potential drawback in utilizing ePTFE material is that it is radially non-compliant compared to a native blood vessel, meaning that the wave propagation of blood, which causes a native blood vessel to expand and contract as pulses of blood flow therethrough, dissipates as it travels through a ePTFE graft. This dissipation of the pulse can lead to various complications, such as compliance mismatch with respect to the host vessel. Unfortunately, to date, it is believed that a radially compliant ePTFE graft that mimics the compliance of a native blood vessel has not been successfully developed. Therefore, there is a need for a self-sealing ePTFE graft that overcomes some or all of the above-mentioned disadvantages.